A conventional hydraulic brake device for a vehicle is disclosed in Japanese Patent No. 56(1981)-10219 and Japanese Patent Laid Open No. 64(1989)-47644. The hydraulic brake device for a vehicle disclosed in the first document mentioned above includes a reservoir for storing brake fluid, a master cylinder connected to the reservoir for generating hydraulic pressure corresponding to the brake operation force, a hydraulic power source connected to the reservoir for generating hydraulic pressure corresponding to a brake operation force, a hydraulic wheel brake, and a pressure control device for effecting anti-lock control. The pressure control device for effecting anti-lock control usually connects the hydraulic wheel brake to the master cylinder. The pressure control device connects the hydraulic wheel brake to the hydraulic power source to control brake fluid flow into the reservoir from the hydraulic wheel brake and to control brake fluid into the hydraulic wheel brake from the hydraulic power source under the anti-lock control condition. The pressure control device comprises a first valve member connected to the wheel brake for switching from the master cylinder to the hydraulic power source, a second valve member for permitting brake fluid flow into the hydraulic wheel brake from the hydraulic power source and a third valve member for permitting brake fluid flow into the reservoir from the hydraulic wheel brake.
The hydraulic brake device disclosed in the second document identified above includes a reservoir for storing brake fluid, a master cylinder connected to the reservoir for generating hydraulic pressure corresponding to the brake operation force, a first hydraulic wheel brake, and a first pressure control device for effecting anti-lock control. The first pressure control device for effecting anti-lock control usually connects the first hydraulic wheel brake to the master cylinder. The first pressure control device connects the first hydraulic wheel brake to the hydraulic power source to control brake fluid flow into the reservoir from the first hydraulic wheel brake and to control brake fluid flow into the reservoir from the hydraulic wheel brake and to control brake fluid flow into the first hydraulic wheel brake from the hydraulic power source under the anti-lock control condition.
The hydraulic brake device also includes a second hydraulic wheel brake and a second pressure control device for effecting anti-lock control. The second pressure control device usually connects the second hydraulic wheel brake to the master cylinder. The second pressure control device connects the second hydraulic wheel brake to the hydraulic power source to control brake fluid flow into the reservoir from the second hydraulic wheel brake and to control brake fluid flow into the second hydraulic wheel brake from the hydraulic power source under the anti-lock control condition.
The first pressure control device comprises a first valve member connected to the first hydraulic wheel brake for switching the master cylinder to the hydraulic power source, a second valve member for permitting the flow of the brake fluid into the first hydraulic wheel brake from the hydraulic power source and a third valve member for permitting brake fluid flow into the reservoir from the first hydraulic wheel brake.
The second pressure control device comprises a first valve member for permitting brake fluid flow into the second hydraulic wheel brake from the hydraulic power source and a second valve member for permitting brake fluid flow into the reservoir from the second hydraulic wheel brake.
Since the master cylinder is connected to the hydraulic wheel brake under normal braking conditions and the hydraulic power source is connected to hydraulic wheel brake under anti-lock brake conditions, the brake operation member of the vehicle can achieve a sufficient stroke under the normal braking condition, and the stroke of the brake operation member does not vary in spite of the reduction and increment of the brake pressure of the wheel brake under the anti-lock braking condition. Further, because two valve members (the first and second valve members) are operated to connect the hydraulic power source to the hydraulic wheel brake, the brake force is prevented from being applied to the hydraulic wheel brake by malfunction of one of the valve members when the brake operation member of the vehicle is not operated.
Because electric control technology which rapidly develops is applied to anti-lock control engineering, a quick reduction mode and a slow increasing mode are mainly used as means for an adjustment of the brake. Since the valve member for permitting the flow of the brake fluid into the hydraulic wheel brake from the hydraulic power source is frequently switched to slowly increase the wheel brake, the valve member causes an operational noise. Further, a valve plunger and a valve seat which form the valve member are highly worn. Therefore, the valve plunger and the valve seat are made of an expensive highly wear proof material.
A hydraulic brake device for a vehicle which applies the brake force to each of the four wheels when the brake operation member of the vehicle is not operated to promote a vehicle stability is disclosed in Japanese Patent Laid Open No. 4(1992)-193658.
The hydraulic brake device for a vehicle disclosed in the above mentioned Japanese Patent Laid Open No. 64(1989)-47644 cannot supply the brake pressure to the front hydraulic wheel brakes when the brake operation member is not operated. Three valve members which are each of the 2-port, 2-position type (2-2 type valve member) must be added to the hydraulic brake device to permit the supply of brake pressure to the front hydraulic wheel brakes when the brake operation member is not operated and to prevent the brake pressure from being applied to the wheel brakes by malfunction of the valve members when the brake operation member of the vehicle is not operated. Therefore, two 3-port, 3-position type valve members (3-3 type valve members), a 3-port, 2-position type valve member (3-2 type valve member) and three 2-2 type valve members must be used in a front hydraulic brake circuit of the prior art. Although each 3-3 type valve member can be replaced by a 2-2 type valve member (normally open type) and a 2-2 type valve member (normally dose type), the front hydraulic brake circuit then requires a 3-2 type valve member and seven 2-2 type valve members. This causes the system to become quite expensive.